1. Field of the Invention
The present invention relates to a magnetic resonance imaging (MRI) apparatus which applies an additional gradient field pulse in addition to a gradient field necessary for imaging.
2. Related Priority Application
This application is based on Japanese Patent Application No. 9-92373, filed Apr. 10, 1997, the content of which is incorporated herein by reference.
3. Related Art
FIGS. 12 and 13 illustrate pulse sequences of a spin echo (SE) scheme and echo planar imaging (EPI) scheme, two typical imaging schemes which are employed in MRI apparatuses. By properly setting the repetition time (TR) and echo time (TE) in those imaging schemes, T1 or T2 of a target nuclide or the density of a target nuclide itself is reflected on the signal value of an image of interest.
Further, an image is provided with specific information by applying an additional gradient field pulse during a period from the application of an excitation pulse to the echo data acquisition.
FIGS. 14A through 14C exemplify additional gradient field pulses. FIG. 14A depicts a pulse sequence in an SE scheme, and FIG. 14B shows a motion probing gradient (Gmo) pulse which is an additional gradient field pulse for enhancing a phase shift in a transverse magnetization spin caused by the minute motion of a molecule including a target nuclide in each image pixel. The Gmo pulse is a pair of gradient field pulses of the same applied time and the same intensity, which are to be applied before and after a radio-frequency (RF) refocusing pulse (180.degree. pulse) and are used to enhance a phase shift in a transverse magnetization spin caused by the diffusion of a molecule including a target nuclide. Such an MPG pulse is disclosed in, for example, a reference document by Stejscal EO, Tanner JE., entitled "Spin Diffusion Measurements: Spin Echoes in the Presence of a Time Dependent Field Gradient, J. of Chemical Physics, 1965, 42: pp 288-299. The imaging scheme which enhances a phase shift in a transverse magnetization spin, originated from the minute motion of a molecule in a same voxel by the application of such additional gradient field pulses, is called IVIM (Intra Voxel Incoherent Motion) imaging.
FIG. 14C shows a flow encode gradient (Gfe) pulse, an additional gradient field pulse for changing the phase of a transverse magnetization spin in proportion to the speed of an imaging target. The specification of U.S. Pat. No. 4,516,075 by Moran discloses an imaging scheme which uses this Gfe pulse.
Each of those additional gradient field pulses like the FEG pulse and Gfe pulse consists of a pulse of one of the frequency encode direction, phase encode direction and slice direction, or consists of a pulse acquired by combining pulses of those directions at a given ratio. The direction for applying an additional gradient field pulse and the intensity of the pulse are set in accordance with the speed and direction of an imaging target to be observed, independently of the image resolution and the slice direction.
Problems of incompleteness which are due to the application of additional gradient field pulses will now be discussed. One of such problems is the occurrence of an unnecessary field distribution.
When the speed of an imaging target of interest is slow, the intensity of an additional gradient field pulse should be increased independently of the gradient field that is necessary for imaging. When an Gmo pulse is used, particularly, its intensity has to be increased significantly; in some cases, Gmo pulses of the maximum gradient field intensity of an MRI apparatus in use are applied for several scores of milliseconds. The application of such an additional gradient field pulse produces an eddy current. The influence of improper adjustment of an eddy field compensation circuit current causes the distribution of an unnecessary field (eddy field) to occur. There are many reports on the distortion, shifting and blurring of images, a change in phase component of images and the like that originate from the occurrence of this unnecessary field distribution.
An unnecessary field distribution also occurs due to a manufacturing error in the allowance of the gradient field coil (more specifically, this error would appear in the section direction). Therefore, the distribution and intensity of an unnecessary field varies from one apparatus to another, resulting in a variation in image quality apparatus by apparatus. This raises a problem for the quality control of commercial MRI apparatuses which are mass-produced.
The above-described occurrence of an unnecessary field distribution is particularly critical in the aforementioned EPI.
IVIM imaging using MPG typically employs EPI to reduce artifacts produced by body motion, CSF (CerebroSpinal Fluid) and pulsation of the bloodstream or the like. From the principle viewpoint, EPI is a sequence especially sensitive to incompleteness, such as the occurrence of an unnecessary field distribution. When MPG is used, for example, as a large gradient field is used in imaging, even the use of a shield type gradient field coil, which is generally said to produce a small eddy current, generates an eddy current too large to be negligible in EPI. The distribution of an unnecessary field (eddy field) due to this eddy current appears as obvious image distortion in EPI even if the residual eddy current is small. This shortcoming is described in another reference document entitled "Correction for Distortion of Images Used to Calculate the Apparent Diffusion Coefficient," Proc. of the 4th Annual Meeting, International Society of Magnetic Resonance in Medicine, 1996, No. 1335. Further, an image obtained by applying an MPG pulse differs in image distortion and shifting from an image acquired without application of an MPG pulse. This leads to such an inconvenience that in comparing both images, it is difficult to specify corresponding positions or that both images cannot be displayed one over the other.
Some imaging schemes acquire extra echo data, besides echoes needed for image reconstruction, as compensation data and compensate various kinds of incompleteness using this compensation data. Here, an unnecessary field distribution produced by a strong additional gradient field pulse reduces the signal value of compensation echo data, which increases artifacts of images like ghost, thus lowering the compensation precision.
As discussed above, application of an additional gradient field pulse undesirably produces an unnecessary field distribution which results in degradation of an image quality, such as image distortion, positional shifting or artifacts produced by reduction of the signal value of compensation echo data.